add a prototype chooser API for integrations

At least go-merkledag and go-graphsync still need a prototype chooser,
and go-ipld-prime-proto provides one, so this simplifies integration
with downstreams.

It's worth noting that we just add PBNode here, while
go-ipld-prime-proto also added RawNode. This is because this dag-pb
module leaves raw nodes out, since those are now in
go-ipld-prime/codec/raw.

The logic is just three lines, but it's still easier to not have to
copy-paste these between different projects.

multicodec.go

@@ -4,7 +4,9 @@ import (
 	"io"
 
 	ipld "github.com/ipld/go-ipld-prime"
+	cidlink "github.com/ipld/go-ipld-prime/linking/cid"
 	"github.com/ipld/go-ipld-prime/multicodec"
+	"github.com/ipld/go-ipld-prime/traversal"
 )
 
 var (
@@ -37,3 +39,14 @@ func Decode(na ipld.NodeAssembler, r io.Reader) error {
 func Encode(n ipld.Node, w io.Writer) error {
 	return Marshal(n, w)
 }
+
+// AddSupportToChooser takes an existing node prototype chooser and subs in
+// PBNode for the dag-pb multicodec code.
+func AddSupportToChooser(existing traversal.LinkTargetNodePrototypeChooser) traversal.LinkTargetNodePrototypeChooser {
+	return func(lnk ipld.Link, lnkCtx ipld.LinkContext) (ipld.NodePrototype, error) {
+		if lnk, ok := lnk.(cidlink.Link); ok && lnk.Cid.Prefix().Codec == 0x70 {
+			return Type.PBNode, nil
+		}
+		return existing(lnk, lnkCtx)
+	}
+}